Last year in the Gulf of Mexico, 15 rigs were set adrift by a succession of powerful hurricanes.
These events prompted the US government’s Minerals Management Service and the United States Coast Guard to work with the oil and gas industry to produce new rig mooring guidelines for the Gulf of Mexico. As a result, demand for anchor-handling boats has never been so high.
The new guidelines require most rigs to have 12, rather than 8, mooring lines. InterMoor’s suction embedded plate anchors (SEPLA), which are cost-effective to manufacture and install, offer increased efficiency at this busy time.
In September 2006, SEPLAs were used to anchor ATP Oil & Gas Corporation’s floating production unit in Mississippi Canyon 711 (formerly Gomez field), Gulf of Mexico. This is their first use as part of a permanent mooring system, although they have been used extensively by companies such as BP, ExxonMobil and Chevron to anchor mobile offshore drilling units in the Gulf of Mexico and off West Africa.
To minimise costs, ATP has converted a semi-submersible drilling rig, the Rowan Midland, to serve as a floating production unit for the expected 10-year life of the field. InterMoor designed and provided a 12-point, taut-leg mooring system using polyester rope and SEPLAs for the unit, which lies in over 920 m of water. The polyester rope costs less and offers better performance than traditional steel cantenary systems, and the SEPLAs are a cost-effective alternative to traditional suction embedded anchors.
SEPLAs are a novel combination of two proven anchoring concepts: suction and plate anchors. The SEPLA system uses a suction follower (similar to a suction anchor) to embed a comparatively small plate anchor relatively deep into the seabed where the more compacted soil has greater retaining capacity.
To ensure their suitability for permanent moorings, the components of the system were subjected to break tests, and each leg was proof loaded in the field to at least 80% of the maximum intact 100-year hurricane load.
Once an anchor is embedded, the suction follower is retracted and used to install additional plate anchors, which makes the system economic to manufacture. Each SEPLA actually uses only a quarter to a third of the steel required for an equivalent suction anchor.
Because the suction follower is returned to surface, installing each SEPLA can take longer than required for conventional suction anchors. However, a typical anchor-handling boat has the deck capacity for just two suction anchors, so must return to shore six times to install the 12 anchors required by most units. In contrast, SEPLAs are about four times smaller than suction anchors, so more anchors can be carried in one trip. Half the number of trips were required to install SEPLAs for Rowan Midland than would have been required for suction anchors.
In fact, the costs for SEPLAs are a third to half those of using suction anchors, and the installation can largely be performed off the critical path. As rates for chartering anchor-handling vessels are at a record high, fewer trips and reduced anchormanufacturing costs are welcome benefits.